Loading of Chip Refiners
The quality of mechanical pulp is very much a function of the energy applied per tonne of production, i.e.: the specific energy. It is therefore very important to be able to adjust the refiner motor loads in order to develop the required specific energy for the pulp quality needed. Most refiners are hydraulically loaded and the normal way to increase refiner motor load is by increasing the axial thrust with more hydraulic pressure. Higher shear force on the fibre is developed resulting in an increase in the torque and in the motor load. Plate gap is reduced.
It has been well established that it might not be possible to reach maximum motor load as defined by the motor capacity. Allison et al. CA 2130277 propose a method to determine the maximum achievable motor load and to operate slightly below this maximum motor load. Owen et al., “A practical approach to operator acceptance of advanced control with dual functionality”, Preprints of Control Systems'98 conference, Porvoo, Finland. Sep. 1-3, 1998, developed a control technique to ensure that the refiner is operated below maximum motor load in order to avoid its sudden drop and also to avoid a sudden drop in the pulp quality. In addition they carried out experiments demonstrating that operating beyond maximum motor load results in fibre cutting and loss of pulp strength properties. Although these two developments represent a significant step towards defining a suitable operating range for a refiner, the fundamental reasons for the difficulties in loading the refiners have not been investigated. As the result, corrective measures are empirical and limited to plate gap adjustments which generally do not correct the problem at the source. These developments apply to certain types of refiners that respond quickly to changes in hydraulic pressure set-points and which are equipped with plate position or plate gap sensors.
Eriksen et al., “Theoretical estimates of expected refining zone pressure in a mill scale TMP refiner” Nordic Pulp & Paper Research Journal (2006), 21(1), 82-89, estimated the mechanical pressure from the pulp in a twin refiner as a function of the amount of fibres covering the bars of the plates. However they never consider the problem of loading the refiner and the loading being related to the mass of fibre in the refining zone.
Nowhere in the literature is there a mention of the possibility that difficulties in loading the refiners could be associated with the mass of fibre in the refining zone, the refining zone becoming full or having an insufficient fibre mass. This, however, is important for monitoring process operation and taking corrective measures.
Pulp Residence Time
The estimation of pulp residence time in the refining zone is a key element for the estimation of the mass of fibre in the refining zone. Pioneering work in this area by Miles, “A Simplified Method for Calculating the Residence Time and Refining Intensity in a Chip Refiner” Paperi ja Puu, Vol. 73/No. 9 (1991), has led to the concept of refining intensity but no effort has been made to use it to estimate the mass of fibre in the refiner and its maximum value.